Aerodynamics of the new Mirage (0.27-0.31 drag coefficient varies with options)

The Mirage / Space Star aerodynamic design is better than average for a small 4-door hatchback body. Mitsubishi lists drag coefficients (Cd) ranging from 0.27 to 0.31, depending on drivetrain and options, with the Japanese and Euro market trims having the lowest Cd:

The most advanced research has gone into the aerodynamics of the Mirage resulting in one the most aerodynamic vehicles in its class. The overall shape has been optimised to fine tune its drag coefficiency to just 0.27 and the sleek body, clam-shell bonnet and tapered roofline create the striking profile of a car engineered from the ground up for fuel economy and CO2 efficiency. (Source)

Notable aero design features (varies by model/options):

Profile taper of the roofline down toward the rear

Drag-reducing "Kammback" type spoiler/roof extension (some models)

Partial lower grille block off (passive -- size of blocked area varies depending on which transmission is equipped: the CVT car has a larger opening than the manual transmission model)

Of course, reducing aero drag isn't just about improving fuel economy. It also results in a quieter cabin (less wind & engine noise) and more engine power in reserve at highway speeds (faster high speed acceleration).

Mitsu provided technical drawings in some of its brochures which we can use to derive frontal area, and ultimately CdA. That figure is more useful for comparing the car's total dragarea to values known for other cars.

Could the Mirage's drag coefficient be made even better?

Yes, at a cost (either financial, or affecting driveability). Here are a few drag-reducing modifications that already exist on other production vehicles that could be adapted to the Mirage:

variable automated grille blocking

reduced ride height

additional underbody panels

a larger "Kammback" drag reducing spoiler

Don't aerodynamics only matter at highway speeds?

People often incorrectly assume that aerodynamic design elements "only matter above 60 MPH" (or some other arbitrary high speed). The fact is, many vehicles burn more than half their fuel to overcome air drag at speeds as low as 25-30 mph (40-50 km/h), including the Mirage at Cd = 0.30.

So, yes, aero is critically important to highway fuel economy. But it also affects city numbers.

Of course the effect is exponentially greater the faster you go. When you double your speed, you cube (x8) the power needed to overcome aero drag.

EDIT: See post #13, below, for calculations that show a difference of 5 MPG between a Mirage with a Cd of 0.27 vs. a Mirage with a Cd of 0.31 cruising a speed of just 50 mph (80 km/h).

Estimating frontal area and total drag (CdA)

Based on dimensional drawings of the car from an Australian brochure, I've calculated the projected frontal area (A) of the car to be 2.04 square meters (21.97 square ft.). (Later I'll post how I got these numbers.)

That means the total drag area (Cd * A) of the lowest drag version of the car (Cd = 0.27) is:
0.551 sq. m (5.93 sq. ft)

How I derived frontal Area (A)

This technical drawing was provided in the Mirage brochure (PDF) from Australia:

It's a decent candidate image to use because it doesn't have perspective distortion we'd typically get in a front-on photograph. Also, Mitsu conveniently provided exact dimensions in millimeters, so we have an accurate reference to figure out the pixels:mm ratio in the image.

First, I enlarged the image to make it easier to work with.

Next, I traced the car's outline with the selection tool, then adjusted the selection so it was centered on the line.

Then I bucket-filled the selected area with black, and deleted everything outside the selected area

I converted the image to black and white (2 colours only) and was left with:

Photoshop has a handy Histogram tool (Image > Histogram) which will tell you the number of pixels for each individual colour in the image

Since it's a 2-colour image only, we can easily see that the black pixels represent only the car's frontal area (153501 pixels, in the image I was working with --- a larger one than the attachment image in this post).

From here on, it's just math:
(A) the diagram dimensions showed the width of the car as 1665 mm.
(B) Using a measurement tool, I found the pixel width of the car in my working image was 457 px.
(C) So we now have a ratio of 1665 mm/457 px, or 3.643 mm/px.
(D) But a pixel is a square, so we have to sqare the number of mm to get 13.274 square mm/px.
(E) Last step: how many pixels in the car? 153747 px * 13.274 mm2/px = 2040811 mm2, or 2.041 square meters.

thanks Metro for that and yes we can add our own under body panels, and maybe able to order the panels from Mitsubishi?

any one figured out how to order these? my ES has no under body cladding and i think it will also help reduce road noise by removing turbulence and adding a layer of material that sound has to go through.

The aeromodders here would be pleased to know that the Mirage comes standard with a grill block.

The bottom grill opening looks big, but actually only 2/3rds of it is open on the manual transmission car. The CVT Mirage has a slightly wider bottom grill opening due to it having a slightly wider radiator. I'll post comparison pics if I can get a picture of a CVT Mirage up close.